Cyclic structure formation method and surface treatment method

ABSTRACT

A periodic structure is to be successively formed over an extensive area with a uniaxial laser beam. Such method includes irradiating a uniaxial laser beam near an ablation threshold to a surface of a material; and executing an overlapped scanning on the irradiated region, so as to cause an ablation by interference between an incident beam and a surface scattered wave along the material surface; increasing the scattered wave; causing an interference at an interval equal to a wavelength of the laser beam, to thereby cause spontaneous formation of a periodic structure. The periodic structure can be made to have a different ripple spacing by changing an incident angle of the laser beam to the material surface. When the laser incident beam has an angle, the ripple spacing can be changed by changing a scanning direction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of forming a periodicstructure and surface treatment, and more particularly to a method ofperiodically forming minute ripples on a surface of a material byirradiating a uniaxial laser beam thereon, and a surface treatment forchanging surface characteristics of a material by irradiating a laserbeam and thereby forming a periodic structure.

2. Description of the Related Art

Recently, development of micromachines, constituted of components thatare a couple of orders of magnitude smaller than those of existingmachines, has been aggressively promoted. While an inertia force such asthe gravity is proportional to a cube of an object size, a surface forceis proportional to a square of the object size. Therefore, whenoperating small parts such as those of a micromachine, an influence ofthe surface force acting between two objects becomes more apparent,rather than an influence of the gravity. Particularly, it is known thata pull-off force (or coagulating force), originating from a surfacetension (or meniscus force) of water produced by condensation ofmoisture in the atmosphere at an interface between two objects, exerts adominant influence to a friction force acting therebetween (Ref.non-patented document 1). It is also known that the pull-off force canbe significantly reduced by minute ripples on the surface (Ref.non-patented document 2).

Also, it has been reported that minute ripples provide remarkable effectin retention of a lubricant and reduction of friction wear, which leadsto an extended life span of the parts (Ref. non-patented document 3),and accordingly there has arisen a demand for development of a techniqueof forming a nanoscale microstructure on a material surface.

Likewise, it is known that irradiating a linearly polarized laser beamof a fluence near an ablation threshold to a polymer results information of a grate-shape minute periodic structure (Ref. non-patenteddocuments 4, 5 and 6). It has also been reported that the same techniqueapplies to a metal and a semiconductor as well, and that changing anirradiation angle can change a ripple spacing of the periodic structure(Ref. non-patented documents 7 and 8).

In all these cases a periodic structure of wavelength order isspontaneously formed, however it takes place only within a laser spot.Accordingly, those methods can only be applied to a limited region. Inthe event that a method of extensively forming such periodic structureon various materials is established, such method will serve to improvetribological characteristics of the material. Further, employing afemtosecond laser beam allows applying the method to small partssusceptible to a thermal effect, as well as to extremely thin parts.

In addition, for example the non-patented document 9 discloses a methodof splitting a high-intensity femtosecond pulse of a titanium-sapphirelaser into two parts, so that the interference of the biaxial laser beamforms a minute periodic structure, and a method of scanning a materialattached to an X-Y stage in synchronization with a repetition frequencyof a laser beam, to thereby form a periodic structure on an entirety ofthe material.

[Non-Patented Document 1]

Yasuhisa Ando, Toshiyuki Tanaka, Jiro Ino and Kazuo Kakuta:Relationships of Friction, Pull-off Forces and Nanometer-scale SurfaceGeometry, Series “C” of JSME (Japan Society of Mechanical Engineers)International Journal, No. 2, Vol. 44(2001), p. 453.

[Non-Patented Document 2]

K. N. G. Fuller and D. Taber,: The effect of surface roughness on theadhesion of elastic solids, Proc. Roy. Soc. Lond., A, 345, (1975) P.327.

[Non-Patented Document 3]

M. Maillat, S. M. Pimenov, G. A. Shafeev and A. V. Simakin, TribolLett., 4, (1998), P. 237.

[Non-Patented Document 4]

P. E. Dyer and R. J. Farley: Periodic surface structures in the excimerlaser ablative etching polymers., Appl. Phys. Lett., 57,8(1990) P. 765.

[Non-Patented Document 5]

H. Hiraoka and M. Sendova: Laser-induced sub-half-micrometer periodicstructure on polymer surfaces, App. Phys. Lett., 64,5(1994) P. 563.

[Non-Patented Document 6]

M. Bolle and S. Lazare: Submicron periodic structures produced onpolymer surfaces with polarized excimer laser ultraviolet radiation,Appl. Phys. Lett., 60,6(1992) P. 674.

[Non-Patented Document 7]

A. E. Siegman, P. M. Fauchet: Stimulated Wood's anomalies onlaser-illuminated surfaces, IEEE J. Quantum Electron., QE-20,8(1986) P.1384.

[Non-Patented Document 8]

Yukimasa Minami and Koichi Toyoda: Incident-angle dependency oflaser-induced surface ripples on metals and semiconductors, Review ofLaser Engineering, No. 12, Vol. 28 (2000), p. 824.

[Non-Patented Document 9]

Ken-ichi Kawamura, Masahiro Hirano and Hideo Hosono: Fabrication ofmicro-gratings on inorganic materials by two-beam holographic methodusing infrared femtosecond laser pulses, Review of Laser Engineering,No. 5, Vol. 30 (2002), p. 244.

However, the method of utilizing the interference of biaxial laser beamsdescribed in the non-patented document No. 9 has the followingdrawbacks. According to the method it is imperative to split the laserbeam to form biaxial laser beams, with additional requirements such assetting an optical path difference to be strictly identical and strictlysynchronizing a laser scanning speed with a ripple spacing of theperiodic structure. Accordingly, control of optical axes is extremelycomplicated, and the apparatus inevitably becomes complicated andexpensive. Besides, the method can only be applied to a flat surfacebecause of utilizing the interference of two optical paths in differentangles, and if a table supporting the material shakes, the ripplespacing of the periodic structure becomes uneven.

Likewise, the methods of forming a periodic structure described in theforegoing non-patented documents 1 through 9 are not appropriate forforming a periodic structure having an accurate ripple spacing over anextensive area through a simplified process, and therefore any practicalapplication of those methods has not been established, since any effectthereof has not been proven yet.

Accordingly, it is an object of the present invention to provide amethod of forming a periodic structure utilizing a uniaxial laser beam,instead of the foregoing biaxial laser beams, on a surface of variousmaterials. It is another object of the present invention to provide asurface treatment technique of irradiating the laser beam on a surfaceof various materials, so as to change the surface characteristicsthereof.

SUMMARY OF THE INVENTION

A first aspect of the present invention provides a method of forming aperiodic structure, comprising irradiating a uniaxial laser beam near anablation threshold to a surface of a material; and executing anoverlapped scanning on the irradiated region, so as to cause an ablationat a section where interference has taken place between an incident beamand a surface scattered wave generated along the material surface, andto thereby cause spontaneous formation of a periodic structure.

Here, while various types of laser beam may be employed including apicosecond or nanosecond pulse laser of a CO₂ laser or YAG laser, it ispreferable to employ a titanium-sapphire laser, for example. Thetitanium-sapphire laser may be employed in a form of an ultra-shortpulse femtosecond laser having, for example, a pulse width of 120 fs,center wavelength of 800 nm, repetition frequency of 1 kHz, pulse energyof 0.25 to 400 μJ/pulse.

A second aspect of the present invention provides the method of forminga periodic structure, wherein the step of irradiating the laser beamincludes setting the laser scanning speed such that 10 to 300 shots oflaser beam irradiation is applied to an identical position, according toa laser spot diameter and a laser oscillating frequency.

A third aspect of the present invention provides the method of forming aperiodic structure, wherein the step of irradiating the laser beamincludes changing an incident angle of the laser beam to the materialsurface, to thereby change a ripple spacing of the periodic structure.

A fourth aspect of the present invention provides the method of forminga periodic structure, wherein the step of irradiating the laser beamincludes irradiating the laser beam at an incident angle, and the stepof executing an overlapped scanning includes changing a scanningdirection of the laser beam so as to change the periodic structure.

A fifth aspect of the present invention provides the method of forming aperiodic structure, wherein the step of irradiating the laser beamincludes changing a direction of polarization so as to change adirection of the periodic structure.

A sixth aspect of the present invention provides the method of forming aperiodic structure, further comprising utilizing a beam expander eitherwith or without a cylindrical lens, thus expanding the laser beam toirradiate a more extensive area.

A seventh aspect of the present invention provides a method of surfacetreatment, comprising forming a grating structure on a surface of amaterial, to thereby change surface characteristics of the material.

An eighth aspect of the present invention provides the method of surfacetreatment, wherein the step of forming the grating structure includesirradiating a laser beam near an ablation threshold to the surface ofthe material; and executing an overlapped scanning on the irradiatedregion, to thereby cause spontaneous formation of the grating structure.

A ninth aspect of the present invention provides the method of surfacetreatment, wherein the step of forming the grating structure includesforming the grating structure so as to overlap in different directions.

A tenth aspect of the present invention provides the method of surfacetreatment, wherein the step of forming the grating structure includesdisposing the grating structure in a mixed layout in differentdirections.

An eleventh aspect of the present invention provides the method ofsurface treatment, wherein the step of forming the grating structureincludes irradiating a laser beam near an ablation threshold having aplurality of pulses of a different direction of polarization to thesurface of the material, such that the pulses do not overlap in time;executing an overlapped scanning on the irradiated region, to therebycause spontaneous formation of the grating structure so as to overlap indifferent directions.

A twelfth aspect of the present invention provides the method of surfacetreatment, wherein the step of forming the grating structure includesirradiating a laser beam near an ablation threshold to the surface ofthe material; and the step of executing an overlapped scanning includeschanging the direction of polarization during the scanning, to therebycause spontaneous formation of the grating structure in a mixed layoutin different directions.

A thirteenth aspect of the present invention provides the method ofsurface treatment, further comprising utilizing a cylindrical lens tocondense the laser beam, thus forming the grating structure in a moreextensive area.

A fourteenth aspect of the present invention provides the method ofsurface treatment, wherein the grating structure is formed with a ripplespacing of 1 μm or less.

A fifteenth aspect of the present invention provides the method ofsurface treatment, wherein the surface characteristics includedustproofness and inhibition of particle adhesion.

A sixteenth aspect of the present invention provides the method ofsurface treatment, wherein the surface characteristics include reductionof friction and friction wear.

A seventeenth aspect of the present invention provides the method ofsurface treatment, wherein the surface characteristics include reductionof wettability.

In the case of irradiating an ultra-short pulse laser (femtosecondlaser) beam to a material surface as the first aspect of the presentinvention, the material is protected from thermal degradation of itsproperties owing to the extremely small pulse width of the laser,because heat conduction barely takes place and hence a substratetemperature close to the irradiation point barely increases, unlike thecase of irradiating a picosecond or nanosecond pulse laser of a CO₂laser or YAG laser. In addition, since a minute periodic structure canbe formed only at a point where the laser beam has been irradiated, thismethod is quite suitable for processing small parts such as those for amicromachine.

More specifically, a thermal diffusion length L_(D) of the laser beamirradiation can be defined as L_(D)=(Dτ₁)^(1/2), where D represents athermal diffusion coefficient of the material, and τ₁ a pulse width ofthe laser. Here, the thermal diffusion coefficient is defined asD=k_(T)/ρc_(p), where k_(T), τ, and c_(p) are thermal conductivity,density, and specific heat, respectively. Accordingly, since the thermaldiffusion length L_(D) is proportional to the square root of the pulsewidth τ₁, irradiating an ultra-short pulse laser beam makes the thermaldiffusion length very short, and when the pulse width is shorter than apicosecond level, the thermal diffusion is reduced to a practicallynegligible level, which is advantageous for processing small parts.

When the laser beam is irradiated on the substrate surface, the laserbeam is scattered by bumps and dips on the substrate, which is definedas a surface scattering. When a linear-polarized laser beam isirradiated on the substrate, an interference takes place between thep-polarization component of the incident beam 1 and the surfacescattered wave along the substrate surface. When the fluence of theincident beam is near the ablation threshold, the ablation takes placeonly at a region of the interference between the incident beam and thesurface scattered wave along the substrate surface. Once the ablationstarts and thereby a surface roughness increases, an intensity of thesurface scattering becomes greater at the next irradiation of the laserbeam, by which the ablation progresses and the interference also occursat a region one wavelength λ farther. By repeating the laser beamirradiation, a periodic structure (grating structure) is spontaneouslyformed, at an interval equal to one wavelength. In this way, irradiationof a uniaxial laser beam can form a periodic structure. Accordingly, theapparatus can be simplified, and hence can be manufactured at a lowercost. Besides, this method provides the advantages that the ripplespacing of the periodic structure is not affected by a vibration of thetable, and that the processing can be performed over a broader range ofworking distance in the direction of the optical axis, such that theperiodic structure can also be formed on a curved surface.

The ripples of the periodic structure spontaneously and sequentiallyformed at a wavelength interval by repetition of the laser beamirradiation, as the second aspect of the present invention, grow to theorder of the wavelength by scores of shots, but irradiation of more than300 shots incurs an excessive thermal effect, thus to make the structurevague. Accordingly, performing the overlapped scanning with 10 to 300shots of laser beam irradiation to an identical position, allows formingthe periodic structure over an extended area.

According to the third aspect of the present invention, changing anincident angle of the laser beam causes a change of the ablationresultant from the interference between the incident beam and thesurface scattered wave along the material surface, thus enabling achange in ripple spacing. Therefore, a periodic structure of a desiredripple spacing can be formed.

According to the fourth aspect of the present invention, changing ascanning direction of the laser beam that has an incident angle causes achange of the ablation resultant from the interference between theincident beam and the surface scattered wave along the material surface,thus enabling a change in periodic structure. Therefore, a differentperiodic structure can be formed simply by changing the scanningdirection, under the same laser beam irradiating conditions. Changingboth the laser scanning direction and the laser beam incident angleallows making a more extensive variety of changes in the periodicstructure.

The fifth aspect of the present invention is based on the fact that theperiodic structure is formed orthogonally to a direction ofpolarization. Accordingly, changing the direction of polarization of theperiodic structure enables changing a direction of the periodicstructure.

According to the sixth aspect of the present invention, expanding thelaser beam by a beam expander, or flattening the expanded laser beam bya cylindrical lens allows executing the laser beam irradiation over abroader area at a time, thus enabling efficient formation of theperiodic structure over an extensive area.

With the surface treatment method according to the seventh aspect of thepresent invention, surface characteristics of the material can bechanged by forming a grating structure on the material surface.

As a specific method a laser beam is irradiated on the material surfaceso as to form the grating structure, in which case, while various typesof laser beam may be employed including a picosecond or nanosecond pulselaser of a CO₂ laser or YAG laser, it is preferable to employ atitanium-sapphire laser, for example. The titanium-sapphire laser may beemployed in a form of an ultra-short pulse femtosecond laser having, forexample, a pulse width of 120 fs, center wavelength of 800 nm,repetition frequency of 1 kHz, pulse energy of 0.25 to 400 μJ/pulse.

In the case of irradiating an ultra-short pulse laser (femtosecondlaser) beam to a material surface, the material is protected fromthermal degradation of its properties owing to the extremely small pulsewidth of the laser, because heat conduction barely takes place and hencea substrate temperature close to the irradiation point barely increases,unlike the case of irradiating a picosecond or nanosecond pulse laser ofa CO₂ laser or YAG laser. In addition, since a minute grating structurecan be formed only at a point where the laser beam has been irradiated,this method is quite suitable for processing small parts such as thosefor a micromachine.

More specifically, a thermal diffusion length L_(D) of the laser beamirradiation can be defined as L_(D)=(Dτ₁)^(1/2), where D represents athermal diffusion coefficient of the material, and τ₁ a pulse width ofthe laser. Here, the thermal diffusion coefficient is defined asD=k_(T)/ρc_(p), where k_(T), ρ, c_(p) are thermal conductivity, densityand specific heat respectively. Accordingly, since the thermal diffusionlength L_(D) is proportional to the square root of the pulse width τ₁,irradiating an ultra-short pulse laser beam makes the thermal diffusionlength very short, and when the pulse width is shorter than a picosecondlevel, the thermal diffusion is reduced to a practically negligiblelevel, which is advantageous for processing small parts.

When the laser beam is irradiated on the substrate surface, the laserbeam is scattered by bumps and dips on the substrate, which is definedas a surface scattering. When a linear-polarized laser beam isirradiated on the substrate, an interference takes place between thep-polarization component of the incident beam 1 and the surfacescattered wave along the substrate surface. When the fluence of theincident beam is near the ablation threshold, the ablation takes placeonly at a region of the interference between the incident beam and thesurface scattered wave along the substrate surface. Once the ablationstarts and thereby a surface roughness increases, an intensity of thesurface scattering becomes greater at the next irradiation of the laserbeam, by which the ablation progresses and the interference also occursat a region one wavelength λ farther. By repeating the laser beamirradiation, the grating structure is spontaneously formed, at aninterval equal to one wavelength. In this way, irradiation of a uniaxiallaser beam can form the grating structure. Forming such gratingstructure on the material surface allows changing one or a plurality ofthe surface characteristics including inhibition of dust or particleadhesion, resistance against friction and friction wear, wettability andso forth.

According to the eighth aspect of the present invention, irradiating alaser beam near an ablation threshold to the surface of the material andexecuting an overlapped scanning on the irradiated region causesspontaneous formation of the grating structure. The grating structurecan be formed for example in an X direction or Y direction, according toa direction of polarization of the laser beam. The ripples formed by thelaser beam irradiation grow to the order of the wavelength by scores ofshots, but irradiation of more than 300 shots incurs an excessivethermal effect, thus to make the structure vague. Accordingly,performing the overlapped scanning with 10 to 300 shots of laser beamirradiation to an identical position allows forming the gratingstructure over an extended area.

The ninth aspect of the present invention is based on the fact thatchanging a direction of polarization of the laser beam allows changing adirection of the grating structure. In the case where, after onceforming a grating structure by irradiating a laser beam near an ablationthreshold and executing an overlapped scanning on the irradiated regionin one direction, a relative angle between the material surface and thedirection of polarization of the laser beam is changed, followed byirradiation of the laser beam near the ablation threshold and overlappedscanning on the irradiated region over the grating structure alreadyformed, a composite grating structure overlapped in a differentdirection can be formed. Accordingly, changing the relative anglebetween the material surface and the direction of polarization of thelaser beam by 90 degrees, when forming the latter grating structure,results in formation of a check patterned grating structure, andchanging the relative angle between the material surface and thedirection of polarization of the laser beam by a desired angle otherthan 90 degrees leads to formation of a bias check patterned gratingstructure.

The tenth aspect of the present invention is also based on the fact thatchanging a direction of polarization of the laser beam allows changing adirection of the grating structure. In the case where, after onceforming a continuous or spaced grating structure in one direction byirradiating a laser beam near an ablation threshold and executing anoverlapped scanning on the irradiated region in one direction, arelative angle between the material surface and the direction ofpolarization of the laser beam is changed, followed by irradiation ofthe laser beam near the ablation threshold on a region adjacent to orspaced from the grating structure already formed and overlapped scanningon the newly irradiated region, a different grating structure can beformed in the region adjacent to or spaced from the first formed gratingstructure. Accordingly, changing the relative angle between the materialsurface and the direction of polarization of the laser beam by 90degrees, when forming the latter grating structure, results in formationof a grating structure in an X direction and the other in a Y direction,disposed in a mixed layout, and changing the relative angle between thematerial surface and the direction of polarization of the laser beam bya desired angle other than 90 degrees leads to formation of the gratingstructures oriented in different directions and disposed in a mixedlayout.

According to the eleventh aspect of the present invention, a laser beamemitted by a laser generator is split into two laser beams with a halfmirror, thus to produce an optical delay in one of the beams. The bothbeams are subjected to a polarizer for polarization in a predetermineddirection, and transmitted to another half mirror, which merges the twobeams polarized in the predetermined direction, so that both beams areirradiated on a material surface. In this way, a laser beam near anablation threshold having a plurality of pulses and including beams of adifferent direction of polarization can be irradiated on the materialsurface, at a predetermined time interval. Then, the overlapped scanningon the irradiated region results in spontaneous and simultaneousformation of a grating structure overlapped in different directions.Accordingly, for example, irradiating a laser beam near an ablationthreshold having a plurality of pulses and directions of polarizationthat are different by 90 degrees at a predetermined time interval, andexecuting an overlapped scanning over the irradiated region, results inspontaneous and simultaneous formation of a check patterned gratingstructure overlapped in an X direction and in Y direction which isorthogonal to the X direction. Also, irradiating laser beams near anablation threshold having a plurality of pulses and directions ofpolarization that are different by a desired angle other than 90 degreesat a predetermined time interval, and executing an overlapped scanningover the irradiated region, results in spontaneous and simultaneousformation of a bias check patterned grating structure intersecting inthe desired angle other than 90 degrees.

The twelfth aspect of the present invention is also based on the factthat changing a direction of polarization of the laser beam allowschanging a direction of the grating structure. In the case where, afteronce forming a grating structure in a predetermined length byirradiating a laser beam near an ablation threshold and executing anoverlapped scanning on the irradiated region, the direction ofpolarization of the laser beam is changed while continuing theirradiation, followed by irradiation of the laser beam near the ablationthreshold on a region adjacent to or spaced from the grating structurealready formed, and overlapped scanning on the newly irradiated region,a different grating structure can be formed in the region adjacent to orspaced from the first formed grating structure. Accordingly, changingthe direction of polarization of the laser beam by 90 degrees whenforming the latter grating structure results in formation of a gratingstructure in an X direction and the other in a Y direction disposed in amixed layout, and changing the direction of polarization of the laserbeam by a desired angle other than 90 degrees and desired times resultsin formation of the desired number of grating structures, oriented inthe desired directions and disposed in a mixed layout.

According to the thirteenth aspect of the present invention, the laserbeam is expanded to a larger diameter laser beam by a beam expander, andthe larger diameter laser beam is then condensed by a cylindrical lens,to be thereby transformed to a narrow and long linear laser beam.Irradiating such linear laser beam on a material surface and executingan overlapped scanning on the irradiated region results in spontaneousformation of the grating structure over an extensive area. Accordingly,a large area grating structure can be formed in a short time.

According to the fourteenth aspect of the present invention, a minutegrating structure with a ripple spacing of 1 μm or less can be easilyprovided, which is unobtainable through an existing mechanicalprocessing. Such surface treatment method can be applied not only to asurface of small parts for a micromachine, but also to a surface ofordinary parts, to thereby change the surface characteristics thereof.

Referring to the fifteenth aspect of the present invention, the gratingstructure reduces a pull-off force originating from a surface tension ofwater produced by condensation of moisture in the atmosphere, andthereby reduces a sticking force of dust or fine particles to theoutermost surface of the material. Therefore, the surfacecharacteristics can be improved in the aspect of inhibiting dust or fineparticle adhesion.

Referring to the sixteenth aspect of the present invention, thereduction of the pull-off force by the grating structure effectivelyserves, in the case of dry friction without a lubricant, to reduce aforce acting on a surface of a mating material in sliding contact withthe outermost surface of the grating structure. This leads toimprovement of the surface characteristics in the aspect of reduction offriction and friction wear. In the case where a lubricant is employed,the reduction effect of friction and friction wear can be equallyachieved, because the grating structure has the functions of retainingand supplementing the lubricant, granting capability of forming a fluidfilm, and preventing adhesion of worn powder.

Referring to the seventeenth aspect of the present invention, thegrating structure increases a ratio of an actual surface area of thematerial against an apparent surface area, and thereby reduces a surfaceenergy than an amount apparently supposed to be. Accordingly, thematerial surface attains reduced wettability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a schematic perspective drawing and a flowchart forexplaining a method of forming a periodic structure according to anembodiment of the present invention;

FIG. 2 is a schematic diagram showing a configuration of an apparatus tobe used for forming a periodic structure according to the embodiment;

FIG. 3( a) is a plan view showing a periodic structure formed on asilicon surface by three-times laser scanning parallel to a direction ofpolarization, by the method of forming a periodic structure according tothe embodiment;

FIG. 3( b) is an enlarged view showing a part of the periodic structureof FIG. 3( a);

FIG. 4( a) is a plan view showing a periodic structure formed on asilicon surface by three-times laser scanning orthogonal to a directionof polarization, by the method of forming a periodic structure accordingto the embodiment;

FIG. 4( b) is an enlarged view showing a part of the periodic structureof FIG. 4( a);

FIG. 5 is a plan view showing a periodic structure formed on a siliconsurface with a laser fluence closest possible to the ablation threshold,by the method of forming a periodic structure according to theembodimen;

FIG. 6 is an enlarged plan view showing a periodic structure formed witha cylindrical lens placed on the silicon surface, by the method offorming a periodic structure according to the embodiment;

FIG. 7 is a plan view showing a periodic structure formed with acylindrical lens placed on a surface of a copper tape, by the method offorming a periodic structure according to the embodiment;

FIGS. 8( a) and 8(b) are enlarged plan views showing a periodicstructure formed on a surface of an aluminum tape and an aluminum foil,respectively, by the method of forming a periodic structure according tothe embodiment;

FIGS. 9( a) and 9(b) are drawings for explaining generation of an S−type interference and an S+ type interference, respectively, between anincident beam and scattered wave.

FIG. 10 is a drawing for explaining a definition of specimen feedingdirection when the incident beam is inclined;

FIGS. 11( a) and 11(b) are enlarged plan views showing a periodicstructure of the S− type and S+ type, respectively, formed by feedingthe copper tape in the direction L;

FIGS. 12( a) and 12(b) are enlarged plan views showing a periodicstructure of the S− type and S+ type, respectively, formed by feedingthe copper tape in the direction R;

FIG. 13 is a line graph showing an incident angle dependency of ripplespacing of a periodic structure formed on silicon and copper;

FIGS. 14( a) and 14(b) are drawings for explaining a formation mechanismof a periodic structure with the feeding direction of L and R,respectively;

FIGS. 15( a) and 15(b) are enlarged schematic perspective views showinga periodic structure formed in an X and Y direction, respectively;

FIG. 15( c) is an enlarged schematic perspective view showing acomposite type periodic structure formed so as to overlap in X and Ydirections;

FIG. 15( d) is an enlarged schematic perspective view showing a periodicstructure formed in X and Y directions in a mixed layout;

FIG. 16 is a schematic diagram showing a configuration of a periodicstructure forming apparatus that forms a periodic structure oriented indifferent directions at a time;

FIG. 17 is a schematic side view showing a rotational sliding testapparatus;

FIGS. 18( a) to 18(d) are plan views respectively showing a periodicstructure of a radial pattern, a concentric circle pattern, a firstspiral pattern and a second spiral pattern;

FIG. 19 is a line graph showing a variation characteristic of a slidingspeed utilized in the rotational sliding test;

FIGS. 20( a) to 20(d) are line graphs showing a sliding speed and afriction coefficient characteristics obtained through the sliding tests,with respect to sliding between mirror surfaces, between the radialpattern and the minor surface, between the concentric circle pattern andthe mirror surface, and between the spiral 1 pattern and the minorsurface, respectively;

FIGS. 21( a) and 21(b) are plan views showing a periodic structure ofthe radial pattern and the concentric circle pattern, respectively, witha friction wear after a loaded sliding test;

FIGS. 22( a) and 22(b) are plan views showing a periodic structure ofthe radial pattern and the concentric circle pattern, respectively,where a friction wear is not caused after a loaded sliding test; and

FIGS. 23( a) to 23(c) are line graphs showing a sliding speed and afriction coefficient characteristics obtained through a disc/discsliding test, with respect to a disc having a concentric circle pattern(23(a)), a radial pattern (23(b)), and a first spiral pattern (23(c)),respectively.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the accompanying drawings, principles for embodyingthe present invention will be described hereunder. FIG. 1 includes aschematic perspective drawing for explaining an underlying mechanism forthe method of forming a periodic structure and of surface treatmentaccording to the present invention, and a flowchart showing a formationprocess of the periodic structure. Referring to the perspective drawingin FIG. 1, when a laser beam 1 is irradiated on a surface of a specimen2, an interference between a p-polarization component 3 of the incidentbeam and a p-polarization component 5 of a surface scattered wave takesplace, to thereby generate a stationary wave 7. Here, numeral 4designates an S-polarization component of the incident beam, and 6 anS-polarization component of the surface scattered wave.

When the fluence of the incident beam is near the ablation threshold ofthe laser, the ablation takes place only at a region of the interferencebetween the p-polarization component 3 of the incident beam and thep-polarization component 5 of the surface scattered wave along thesubstrate surface (12 of the flowchart). Once the ablation starts andthereby a surface roughness increases, an intensity of the surfacescattering becomes greater at the next irradiation of the laser beam(13), by which the ablation progresses and the interference also occursat a region one wavelength λ farther. When the incident beam is linearlypolarized, repetition of the laser beam irradiation causes aninterference at an interval equal to the wavelength λ of the incidentbeam (14), and thus a periodic structure is spontaneously formed (15).

The ripples of the periodic structure grow to the order of thewavelength by 10 to 300 shots of irradiation, but irradiation of morethan 300 shots makes the ripple structure vague. Accordingly, performingthe overlapped scanning with 10 to 300 shots of laser beam irradiationto an identical position allows forming the grating structure over anextended area. For the scanning, either of the table supporting thespecimen 2 or the laser emitter may be moved.

FIG. 2 is a schematic diagram showing a configuration of an apparatus tobe used for forming the periodic structure 20. It is to be noted inadvance that specific numerical values indicated in the followingpassages are only exemplarily stated for clearer understanding of thedescription, and not for delimiting purpose. A titanium-sapphire laserbeam 1 (pulse width: 120 fs, center wavelength λ: 800 nm, repetitionfrequency: 1 kHz, pulse energy E: 0.25 to 400 μJ/pulse) generated by atitanium-sapphire laser generator 21, upon being set such that the pulseenergy is adjustable with a ½ wavelength plate 22 and a polarizing beamsplitter 23, was irradiated on a surface of the specimen 2 on an X-Y-θstage 25, through a lens (focal length: f=100 mm) 24. The resolution ofthe X-Y-θ stage 25 may be set as desired, and was actually set at 2 μmas an example. The X-Y-θ stage 25 was set to move the specimen 2 at aspeed of 0.25 mm/s (125 pps) so as to perform overlapped irradiation ofthe laser beam 1 on the specimen 2, and to cause the ablation by theinterference between the incident beam and the surface scattered wavealong the surface, for successive formation of a periodic structure.

A scanning speed for the specimen 2 is set according to a spot diameterand intensity of the laser beam 1. The incident angle θ to the specimen2 was set at 0 degree, 15 degrees, 30 degrees and 45 degrees. As thespecimen 2, a silicon, a copper tape and an aluminum tape of 50 μm inthickness, as well as an aluminum foil of 15 μm in thickness wereemployed. For observation of the periodic structure thus obtained, alaser microscope and an atomic force microscope (AFM) were utilized.

[Periodic Structure on Silicon (Incident Angle 0 Degree)]

A surface of the silicon substrate, employed as the specimen 2, wasscanned three times by the laser beam 1 near the ablation threshold,through a piano-convex lens 24 of a focal length of 100 mm, to therebyform a periodic structure. FIGS. 3( a) and 3(b) show the periodicstructure formed with the scanning direction of the laser beam 1 and thedirection of polarization set in parallel. FIGS. 4( a) and 4(b) show theperiodic structure formed with the direction of polarization rotated by90 degrees. FIGS. 3( a) and 4(a) show an overall appearance, while FIGS.3( b) and 4(b) show enlarged images of the periodic structure. Referringto FIGS. 3( a) and 4(a), the laser beam irradiation was suspendedhalfway of the second scan, for visual understanding that the scanningwas performed three times. These periodic structures are all orientedorthogonally to the direction of polarization. The ripple spacing of theperiodic structure is approx. 700 nm, which is slightly shorter than thelaser wavelength λ (800 nm). Overlapped sections of the scanning do notpresent a significant disorder.

FIG. 5 shows a periodic structure formed with a laser fluence loweredclosest possible to the ablation threshold, so as to restrain theablation as much as possible. The ripple spacing is 795 nm, which iswell in accordance with the laser wavelength λ (center wavelength 800nm).

In order to form the periodic structure over a more extensive area, abeam expander was employed to expand the laser beam, and also acylindrical lens having a focal length of 100 mm was employed. As aresult, the periodic structure has been formed in a width exceeding 2 mmby one scan. Such periodic structure is shown in FIG. 6. The ripplespacing is 700 nm, which is similar to that of the periodic structureformed by the laser beam near the ablation threshold through thepiano-convex lens as FIG. 3( b).

Upon irradiating a white light on the periodic structure formed throughthe cylindrical lens, a spectroscopic capability has been confirmed, bywhich it has been proven that the periodic structure is formed atregular intervals over an extensive area.

[Periodic Structure on Copper Tape (Incident Angle 0 Degree)]

Upon forming a periodic structure on the copper tape through thecylindrical lens having the focal length of 100 mm, a similar periodicstructure to that formed on the silicon has been obtained, over a widthexceeding 2 mm by one scan. However in the case of the copper tape,setting the work distance at 99 mm, which is 1 mm shorter than the focallength, resulted in forming a relatively excellent periodic structure.Also, more than three times of pulse energy (E=400 μJ/pulse) wasrequired with respect to the case of the silicon (E=100 μJ/pulse). FIG.7 shows the periodic structure formed on the copper tape. The ripplespacing of the periodic structure is approx. 700 nm, which issubstantially the same as that of the silicon. Upon irradiating a whitelight on the periodic structure formed through the cylindrical lens, aspectroscopic capability has been confirmed as with the silicon.

[Periodic Structure on Aluminum Tape and Foil (Incident Angle 0 degree)]

Upon forming a periodic structure on the aluminum tape and the aluminumfoil through the beam expander and the cylindrical lens having the focallength of 100 mm, periodic structures respectively shown in FIGS. 8( a)and 8(b) have been obtained. The ripple spacing of the periodicstructure on the aluminum tape and the aluminum foil is 600 nm. Uponirradiating a white light on both of the periodic structures, aspectroscopic capability has been confirmed. Also, thermal effect hasnot been observed on a rear face of the aluminum foil of 15 μm inthickness.

[Incident Angle Dependence and Scanning Direction Dependence of thePeriodic Structure]

When a laser beam 1 of a wavelength λ is irradiated on the specimen 2 atan incident angle θ, two types of interference take place as shown inFIGS. 9( a) and 9(b). Hereinafter, in order to distinguish theseinterferences, the interference with a wider interval as shown in FIG.9( a) is defined as an “S− type” interference, and the interference witha narrower interval as shown in FIG. 9( b) as an “S+type” interference.When the respective intervals are designated by X_(S−) and X_(S+), theinterval X_(S−) in the case of FIG. 9( a) is obtained by the followingformula:

$\begin{matrix}{X_{s -} = \frac{\lambda}{1 - {\sin\;\theta}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

The interval X_(S+) in the case of FIG. 9( b) is obtained by thefollowing formula:

$\begin{matrix}{X_{s +} = \frac{\lambda}{1 + {\sin\;\theta}}} & \left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack\end{matrix}$

Upon irradiating the laser beam 1 on the specimen 2 through the beamexpander and the cylindrical lens at the incident angle θ of 15 degrees,30 degrees and 45 degrees, a periodic structure having differentintervals has been formed in an overlapped state, on the silicon and thecopper tape. Especially when moved in a feed direction L shown in FIG.10, both of the S− type with a wider interval and the S+ type with anarrower interval have been formed with a clear contrast. On the otherhand, when moved in a feed direction R, formation of the S− type hasbeen immature in comparison with the case of the feed direction L.

FIGS. 11( a) and 11(b) show a periodic structure formed on the coppertape at the incident angle of 45 degrees and in the feed direction L.The images of FIGS. 11( a) and 11(b) have been shot toward a same pointwith different focus, in which the S− type periodic structures can beclearly observed, and can also be vaguely seen when observing the S+type periodic structure, since both of the S− and S+ type periodicstructures are formed at the same position when moved in the feeddirection L.

FIGS. 12( a) and 12(b) show a periodic structure formed on the coppertape at the incident angle of 45 degrees and in the feed direction R. Inthis feed direction R also the both types of periodic structures havebeen obtained, however the S− type periodic structure is ofteninterrupted, and can barely be seen when observing the S+ type periodicstructure. Accordingly, it is proven that the S− type periodic structureis more clearly formed when moved in the feed direction L.

FIG. 13 is a line graph showing a relationship between the incidentangle and ripple spacing of a periodic structure formed on the siliconand the copper tape, along with theoretical values.

A reason that the S− type periodic structure having a wider ripplespacing is more clearly formed when moved in the feed direction L can beexplained as follows. In the case of the feed direction L, the S− typeperiodic structure 31 is first formed on a plane surface as shown inFIG. 14( a), and the S+ type periodic structure 32 is formed so as tooverlap when the specimen 2 is moved. By contrast, in the case of thefeed direction R, since the S+ type periodic structure 32, which has anarrower ripple spacing, is first formed on the plane surface as shownin FIG. 14( b), there is no longer enough room for the S− type periodicstructure to be clearly formed.

As a result of forming minute periodic structures utilizing thefemtosecond laser beam of a fluence near the ablation threshold on thesilicon, copper tape and aluminum tape as above, the following factshave been confirmed.

1. On silicon, copper and aluminum, a periodic structure can be formedover an extensive area by irradiating a uniaxial laser beam on a surfaceof the specimen through a cylindrical lens and performing the scanningthereon.

2. The periodic structure has an incident angle dependency, and atheoretical value of the interval is λ/(1±sin θ).

3. The periodic structure has a scanning direction dependency, and theS− type periodic structure is more clearly formed in the feed directionL.

4. Different periodic structures are formed through a same mechanism,i.e. formed by interference between an incident beam and a surfacescattered wave.

In addition, with respect to different specimens from the silicon,copper tape and aluminum tape, similar periodic structures have beenobtained to those formed on the silicon, copper tape and aluminum tape.

Now description will be given on a method of surface treatment accordingto the present invention.

As already described in details, irradiating a laser beam on a materialsurface and perform a scanning with the irradiating beam leads toformation of a periodic structure, according to the present invention.Here, when a direction of polarization of the laser beam is set in adirection Y, a periodic structure 8 _(X) oriented in a direction X isobtained as shown in FIG. 15( a), while setting the direction ofpolarization of the laser beam in the direction X results in formationof a periodic structure 8 _(Y) oriented in a direction Y, as shown inFIG. 15( b).

Also, changing a direction of polarization of the laser beam allowschanging a direction of the periodic structure. Based on this, in thecase where, after once forming a periodic structure 8 _(X) oriented inone direction as shown in FIG. 15( a) by irradiating a laser beam nearan ablation threshold and executing an overlapped scanning on theirradiated region in one direction, a relative angle between thematerial surface and the direction of polarization of the laser beam ischanged, followed by irradiation of the laser beam near the ablationthreshold and overlapped scanning on the irradiated region over theperiodic structure already formed so as to form a periodic structure 8_(Y) in a different direction, a composite grating structure 8 _(Z)overlapped in a different direction can be formed.

Accordingly, changing the relative angle between the material surfaceand the direction of polarization of the laser beam by 90 degrees asshown in FIG. 15( c), when forming the latter periodic structure,results in formation of a check patterned periodic structure 8 _(Z), andchanging the relative angle between the material surface and thedirection of polarization of the laser beam by a desired angle otherthan 90 degrees leads to formation of a bias check patterned periodicstructure.

Referring now to FIG. 15( d), in the case where, after once forming aperiodic structure 8 _(X) in one direction by irradiating a laser beamnear an ablation threshold and executing an overlapped scanning on theirradiated region in one direction, a relative angle between thematerial surface and the direction of polarization of the laser beam ischanged, followed by irradiation of the laser beam near the ablationthreshold on a region adjacent to or spaced from the periodic structure8 _(X) already formed and overlapped scanning on the newly irradiatedregion, another periodic structure 8 _(Y) can be formed in a differentdirection in the region adjacent to or spaced from the first formedperiodic structure 8 _(X). Accordingly, changing the relative anglebetween the material surface and the direction of polarization of thelaser beam by 90 degrees, when forming the latter periodic structure,results in formation of a periodic structure 8 _(X) in an X directionand the other 8 _(Y) in a Y direction, disposed in a mixed layout, andchanging the relative angle between the material surface and thedirection of polarization of the laser beam by a desired angle otherthan 90 degrees leads to formation of the periodic structures orientedin different directions and disposed in a mixed layout.

Also as already stated, based on the fact that changing the direction ofpolarization of the laser beam leads to a change in the orientation ofthe periodic structure, a grating structure overlapped in differentdirections as shown in FIG. 15( c) can be formed through one processutilizing a periodic structure forming apparatus 40 as shown in FIG. 16.The periodic structure forming apparatus 40 of FIG. 16 emits a laserbeam L₀ generated by a titanium-sapphire laser generator 41, so that thelaser beam L₀ is totally reflected by a mirror 42, and split by a halfmirror 43 into a reflected laser beam L₁ and a transmitted laser beamL₂. Then the reflected laser beam L₁ is totally reflected by mirrors 44,45, so as to produce an optical delay 46 on the transmitted laser beamL₂. This optical delay 46 includes mirrors 47, 48. Laser beams L₃, L₄produced by polarizing the laser beams L₁, L₂ with polarizer 49, 50 areprovided to a half mirror 51, so that the half mirror 51 merges thepolarized laser beams L₃, L₄ and irradiates through a lens 52 to asurface of a material 54 set on an X-Y table 53. In this way, the laserbeams L₃, L₄ near the ablation threshold having a plurality of pulsesand different directions of polarization can be irradiated to thesurface of the material 54 at a determined time interval. Then executingan overlapped scanning over the irradiated region results in spontaneousand simultaneous formation of a periodic structure 8 _(Z) overlapped indifferent directions as shown in FIG. 15( c).

Accordingly, for example, irradiating the laser beams L₃, L₄ near theablation threshold having a plurality of pulses and directions ofpolarization that are different by 90 degrees at a predetermined timeinterval, and executing an overlapped scanning over the irradiatedregion, results in spontaneous and simultaneous formation of a checkpatterned periodic structure 8 _(Z) as shown in FIG. 15( c), in whichthe periodic structure 8 _(X) oriented in an X direction and theperiodic structure 8 _(Y) oriented in a Y direction which is orthogonalto the X direction are overlapping. Also, irradiating laser beams nearan ablation threshold having a plurality of pulses and directions ofpolarization that are different by a desired angle other than 90 degreesat a predetermined time interval, and executing an overlapped scanningover the irradiated region, results in spontaneous and simultaneousformation of a bias check patterned grating structure intersecting inthe desired angle other than 90 degrees.

Descriptions will now be made on changes in surface characteristics ofmaterials resultant from the formation of the periodic structure. As amaterial for examining the surface characteristics, a silicon having athickness of 50 μm was employed. For irradiation, a titanium-sapphirelaser beam, for example having a pulse width of 120 fs, centerwavelength of 800 nm, repetition frequency of 1 kHz, pulse energy of 100μJ/pulse was expanded by a beam expander and condensed by a cylindricallens. Such laser beam was irradiated on the silicon surface, and anoverlapped scanning was performed on the irradiated region, at ascanning speed of 0.25 mm/s. As a result, a periodic structure of 0.7 μmin ripple spacing and 0.2 μm in depth has been formed.

Upon comparing a fine particle (glass particle of 20 μm in diameter)adhesion characteristic of the silicon surface where such periodicstructure has been formed, with the fine particle adhesioncharacteristic of a mirror-surfaced silicon, a result shown in table 1has been obtained.

TABLE 1 Inhibition of fine particle adhesion The present inventionMirror surface Area 115 mm² 68.5 mm² Number of particles 0 260 adheredAdhesion density 0/mm² 3.8/mm²

In view of the table 1, which evidently shows the inhibition effectprovided by the periodic structure agaisnt fine particle adhesion on thesilicon surface, it is obvious that a pull-off force has been reduced.In a micromachine, since a weight of itself is extremely small, thepull-off force exerts a dominant influence over a friction force. Sinceforming a periodic structure according to the present invention reducesthe pull-off force, it becomes possible to reduce a friction force of amicromachine. Also, the reduction of a friction force results inreduction of friction wear.

Further, in addition to micromachines, the present invention can besuitable applied to precision parts such as a crank shaft or piston ringof an automobile engine, for reduction of friction and friction wear ona surface of those parts, because the periodic structure has thefunctions of retaining and supplementing the lubricant, grantingcapability of forming a fluid film, and preventing adhesion of wornpowder.

WORKING EXAMPLE

An effect of the periodic structure according to the present inventionto sliding characteristic will be described as under.

FIG. 17 is a schematic side view showing a sliding test apparatus 60,employed for sliding tests of disc-shaped test pieces on which theforegoing periodic structures are provided. The sliding test apparatus60 includes a base 61 rotatably supporting a test piece table 63 via abearing 62, and the test piece table 63 is provided with a recessedportion for retaining a fixed test piece 64 therein. On the base 61,also a pillar 65 is erected, on which a load cell 66 is disposed suchthat a rotating torque of the test piece table 63 is applied thereto viaa cantilever 67. Further, a rotating test piece 68 is disposed so as tooppose the fixed test piece 64, and pure water 69 is filled in therecessed portion of the test piece table 63, so that the pure water 69is interposed between sliding surfaces of the fixed test piece 64 andthe rotating test piece 68. A load 70 is applied to the rotating testpiece 68, and a rotative driver (not shown) causes a rotating motion 71.

The disc-shaped test pieces are made of an ultra-hard alloy, on whichvarious ring-shaped periodic structures have been formed as shown inFIGS. 18( a) to 18(d). FIG. 18( d) shows a radial periodic structure,FIG. 18( b) shows a concentric circle pattern radial periodic structure,FIG. 18( c) shows a first spiral periodic structure, and FIG. 18( d)shows a second spiral periodic structure. The first spiral periodicstructure of FIG. 18( c) and the second spiral periodic structure ofFIG. 18( d) are different in the direction (angle) of the spiralpattern.

As the rotating test piece 68, a hardened stainless steel according toSUS440C was employed, while an ultra-hard alloy was employed as thefixed test piece 64. Surface roughness of the respective test pieceswere set at Rmax 0.05 μm or lower, and flatness of 1 band or less by anoptical flat and red LED. On the surface of the fixed test pieces 64,the four patterns of periodic structure have been formed in an annularregion having an inner diameter of 9.75 mm and an outer diameter of16.25 mm. Depth of grooves is approx. 0.2 μm, and the groove angle ofthe spiral is 45 degrees with respect to a radius.

One of the rotating test pieces 68 is of a ring shape having an innerdiameter of 10 mm and an outer diameter of 16 mm, with a mirror surfaceof the above precision, and the other is of a disc shape having an outerdiameter of 16 mm, with both of which ring/disc tests and disc/disctests simulating a thrust bearing were performed. A sliding test ofmirror surfaces was also performed for comparison purpose. All the testswere performed in a clean booth (corresponding to class 100) at a roomtemperature of 23 degree centigrade, and the test pieces wereultrasonically cleaned with ethanol and pure water.

The load was constantly set at 10N, and the sliding speed was graduallyreduced from 1.2 m/s to 0.15 m/s every minute. FIG. 19 shows a changingpattern of the sliding speed on the part of the rotating test piece 68.Here, the sliding speed is taken at a position corresponding to anaverage diameter of 13 mm on the rotating test piece 68 employed information of the ring-shaped periodic structure and the ring/discsliding test. Thus, on the sliding test apparatus 60 according to FIG.17, the sliding (rotating) speed of the rotating test piece 68 wasreduced with the lapse of time, so that a change of friction coefficientbetween the fixed test piece 64 and the rotating test piece 68 could bemore clearly recognized.

After the test, a friction coefficient μ has been worked out based on asliding torque, by the following formula 3.

$\begin{matrix}{\mu = \frac{M}{Wr}} & \left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack\end{matrix}$

where M represents a sliding torque, and W a load. The “r” represents anaverage radius of 6.5 mm, of the rotating test pieces 68 employed forformation of the ring-shaped periodic structure and the ring/disc tests.Although applying the formula 3 based on r of 6.5 mm in the disc/disctest does not make sense from a viewpoint of physics, the value of r=6.5mm has been utilized for easier understanding of an effect in a centralportion of the rotating test piece 68 in comparison with the result ofthe ring/disc tests.

[Test Result]

[Ring/Disc Test]

FIGS. 20( a) to 20D are line graphs showing a sliding test result inwhich the fixed test pieces having the minor surface (20(a)), a periodicstructure of the radial pattern (20(b)), a periodic structure of theconcentric circle pattern (20(a)) and a periodic structure of the firstspiral pattern (20(d)) were respectively used.

In the case of the test piece with the minor surface of FIG. 20( a), thefriction coefficient sharply increased immediately upon starting thesliding test. With respect to the test piece having the radial patternperiodic structure of FIG. 20( b), the friction coefficientsignificantly decreased in comparison with the test piece with themirror surface. With respect to the test piece having the concentriccircle pattern periodic structure as FIG. 20( c), a visible fluidlubrication region has not been observed. In the case of the firstspiral pattern periodic structure as FIG. 20( d), an intermediatecharacteristic between the radial pattern periodic structure and theconcentric circle periodic structure has been observed, in the aspectsof the fluid lubrication region and conformability in mixed lubrication.With respect to the test piece with the second spiral periodicstructure, since the pattern serves as a pump to discharge the purewater from a central portion toward a peripheral portion because of thesliding motion, the friction coefficient suddenly increased up to higherthan 0.5 once the pure water was completely discharged.

In the case of the test piece with the mirror surface of FIG. 20A, thefriction coefficient sharply increased immediately upon starting thesliding test. With respect to the test piece having the radial patternperiodic structure of FIG. 20B, the friction coefficient significantlydecreased in comparison with the test piece with the mirror surface.With respect to the test piece having the concentric circle patternperiodic structure as FIG. 20C, a visible fluid lubrication region hasnot been observed. In the case of the first spiral pattern periodicstructure as FIG. 20D, an intermediate characteristic between the radialpattern periodic structure and the concentric circle periodic structurehas been observed, in the aspects of the fluid lubrication region andconformability in mixed lubrication. With respect to the test piece withthe second spiral periodic structure, since the pattern serves as a pumpto discharge the pure water from a central portion toward a peripheralportion because of the sliding motion, the friction coefficient suddenlyincreased up to higher than 0.5 once the pure water was completelydischarged.

For evaluating discharging capability of worn powder, the load wasincreased to 100N, which is ten times as great as the load of anordinary test, under which traces of wear were formed on the periodicstructures, and observation thereof was performed. FIG. 21( a) shows astate of the radial pattern periodic structure, and FIG. 21( b) that ofthe concentric circle periodic structure. While the radial patternperiodic structure has been fully filled with the worn powder, groovesof the concentric circle periodic structure still remain uncovered withthe worn powder, though scale-shaped worn particles are formed on theperiodic structure.

FIGS. 22( a) and 22(b) show a state of the periodic structure of thesame test pieces as FIGS. 21( a) and 21(b), but at a portion where thetrace of wear is not produced. On the radial pattern periodic structureof FIG. 22( a) not much worn powder is observed, while a multitude ofworn powders of approx. 100 nm is stuck on the concentric circleperiodic structure of FIG. 22( b). In view of this, it is understoodthat the worn waste has barely moved from where it was produced in theradial pattern periodic structure, while the worn powder is dischargedwith the fluid by the grooves on the concentric circle periodicstructure.

[Disc/Disc Test]

FIGS. 23( a) to 23(c) are line graphs showing changes in frictioncoefficient of the test pieces having the concentric circle periodicstructure (23(a)), the radial pattern periodic structure (23(b)), andthe first spiral pattern periodic structure (23(c)), respectively. Inall these test samples, mutual sliding of the minor surfaces takes placein a central portion thereof. However with respect to the concentriccircle periodic structure in particular, which does not practicallyproduce a load capacity, the greatest friction coefficient was presentedsince a sliding friction readily takes place.

Based on the test result with respect to the foregoing periodicstructures, the following conclusions have been reached.

1. A periodic structure having a sub-micron interval and groove depthspontaneously formed by irradiation of a femtosecond laser beamsignificantly reduces a friction coefficient.

2. A radial pattern periodic structure can improve a load capadity of afluid lubrication film, and presents a fluid lubrication region in abroadest condition range at a ring/disc sliding test.

3. A concentric circle periodic structure has a prominent worn powderdischarging capability and prevents adhesion of the worn powder, andtherefore shows a lowest friction coefficient in a mixed lubricationregion at a ring/disc sliding test.

4. A spiral pattern periodic structure also can improve a load capacityof a fluid lubrication film, and presents a fluid lubrication region ina broadest condition range at a disc/disc sliding test.

1. A method of forming a periodic structure, comprising: irradiating asurface of a material with a linearly polarized single laser beam of afemtosecond laser, of which a fluence is above but nearly as low asablation threshold; and executing an overlapped scanning in which thelaser beam is scanned on the material surface with a laser scanningspeed being set such that the number of pulses of the laser beamirradiated on an identical position of the material surface is within arange of 10 to 300, so as to cause the ablation on the material surfaceat a section where interference has taken place between a p-polarizationcomponent of an incident beam and a p-polarization component of asurface scattered wave generated along the material surface, and tothereby form a periodic structure on the material surface, wherein theperiodic structure has ripples spacing near a wavelength of the incidentbeam in a direction perpendicular to a polarization direction of theincident beam.
 2. The method according to claim 1, wherein the step ofirradiating the laser beam includes changing an incident angle of thelaser beam to the material surface, to thereby change a ripple spacingof the periodic structure.
 3. The method according to claim 1, whereinthe step of irradiating the laser beam includes irradiating the laserbeam at an incident angle, and the step of executing the overlappedscanning includes changing a scanning direction of the laser beam so asto change the periodic structure.
 4. The method according to claim 1,wherein the step of irradiating the laser beam includes changing adirection of polarization so as to change a direction of the periodicstructure.
 5. The method according to claim 1, further comprisingutilizing a beam expander either with or without a cylindrical lens,thus expanding the laser beam to irradiate a more extensive area.
 6. Themethod according to claim 1, wherein the periodic structures indifferent directions are formed on the material surface in state ofoverlapping each other.
 7. The method according to claim 1, wherein theperiodic structures in different directions are formed on the materialsurface in state of being adjacent to or spaced from each other.
 8. Themethod according to claim 6, wherein the laser beam is split into twolaser beams having a different direction of polarization to each other,and wherein the laser beams are irradiated on the material surface at apredetermined time interval such that the laser beams do not overlapeach other.
 9. The method according to claim 7, wherein the direction ofpolarization of the laser beam is changed during the scanning.
 10. Themethod according to claim 1, wherein the laser beam is condensed by acylindrical lens.
 11. The method according to claim 1, wherein thematerial surface formed with the periodic structure has characteristicsincluding dust proofness and inhibition of particle adhesion.
 12. Themethod according to claim 1, wherein the material surface formed withthe periodic structure has characteristics including reduction offriction and friction wear.
 13. The method according to claim 1, whereinthe material surface formed with the periodic structure hascharacteristics including reduction of wettability.